Liquid crystal displays use liquid crystal switching to visualize the polarization state, and based on the display principle, they use a polarizing plate including a polarizer and transparent protective films provided on both sides of the polarizer with an adhesive layer interposed therebetween. For example, iodine polarizers made of stretched polyvinyl alcohol to which iodine is adsorbed have high transmittance and high degree of polarization. Therefore, they are most popular polarizers widely used. Triacetylcellulose or the like has high water-vapor permeability and therefore is used for transparent protective films.
Image displays such as liquid crystal displays as an application of the polarizing plate are used under various environments. Therefore, it is demanded that the polarizing plate should have durability such as heat resistance under high temperature environment or humidity resistance under high humidity environment. In recent years, it has been demanded that durability should be satisfied particularly in mobile applications such as cellular phones.
In TV applications, image display quality has been highly improved, and therefore, screen uniformity (unevenness) requirements are very strict. For example, polarizing plates are required to have water resistance to such an extent that their properties and form will not change even under humidified conditions such as conditions causing dew condensation. However, the use of a triacetylcellulose film as a transparent protective film is more likely to cause unevenness, because it has high water-vapor permeability and high photoelastic coefficient. On the other hand, when a polyethylene terephthalate film or a norbornene-based film, which has low water-vapor permeability, is used as a transparent protective film in a polarizing plate, water cannot be released from the polarizer, so that a steam-heated state is brought about, which contrarily degrades the optical properties.
The transparent protective film used in a polarizing plate is bonded to the polarizer with an adhesive. Therefore, the process of preparing a polarizing plate has a problem in which knick defects may occur when the polarizer and the transparent protective film are bonded together. Knick defects are defects of local irregularities formed at the interface between the polarizer and the transparent protective film. Against such knick defects, there is proposed a method that includes using, as a polarizer, a polyvinyl alcohol-based film with a controlled water content and with its surface treated with a calendar roll under specific conditions, and laminating the film and a transparent protective film (Patent Literature 1). Knick defects are particularly easy to occur when a polyvinyl alcohol-based resin having an acetoacetyl group is used for the polyvinyl alcohol-based adhesive.
Patent Literature 1: JP-A No. 10-166519